Olefin polymerization catalysts prepared by combining an organoaluminum component with a solid second component containing magnesium, titanium and halogen are well known in the art. It is also well known that the activity of such catalysts, as well as their ability to produce stereoregular polymers, can be enhanced by incorporating an electron donor (Lewis base) into the solid second component. The addition of an electron donor to the catalyst system independently of the solid second component is also known to enhance the stereospecific character of these catalysts. When the electron donor is added separately from the solid second component, it may be complexed in whole or in part with the organoaluminum component. When an electron donor is added separately from the second catalyst component, it is sometimes referred to as a selectivity control agent or an outside electron donor. The electron donor incorporated into the second catalyst component is referred to as an inside electron donor.
More specifically, U.S. Pat. No. 4,414,132 discloses an olefin polymerization catalyst for the production of polymers of high isotacticity comprising (1) an organoaluminum compound, (2) a selectivity control agent, and (3) a solid composition obtained by halogenating a magnesium compound of the formula MgR'R", wherein R' is an alkoxide or aryloxide group and R" is an alkoxide or aryloxide group or halogen, with a halogenated tetravalent titanium compound in the presence of a halohydrocarbon and an electron donor, and subsequentlycontacting the halogenated product with additional tetravalent titanium compound. According to the reference, the organoaluminum compound and the selectivity control agent may be used separately, or partially or completely complexed with each other. The solid component obtained by treating the magnesium compound with the halogenated titanium compound is referred to in the reference as the "procatalyst", and the organoaluminum compound, whether used separately or partially or completely complexed with the selectivity control agent, is referred to as the "cocatalyst". An electron donor is employed as the "selectivity control agent", and this term is applied to such electron donor whether it is used separately or partially or completely complexed with the organoaluminum compound.
U.S. Pat. No. 4,535,068 discloses that the productivity of the olefin polymerization catalyst prepared in accordance with U.S. Pat. No. 4,414,132 can be improved as much as 20 percent if the product obtained by halogenating the magnesium compound with the halogenated tetravalent titanium compound in the preparation of the "procatalyst" of that reference is treated with a carboxylic acid halide prior to or at the same time that it is treated with additional tetravalent titanium compound. However, as in the case of the stereospecific catalyst of U.S. Pat. No. 4,414,132, as the concentration of selectivity control agent in the catalyst is increased in an attempt to increase the amount of stereoregular polymer produced, the catalyst undergoes increasing decline in activity. This decline in activity is accentuated as the temperature of polymerization is increased. Thus, this catalyst, like the catalyst of U.S. Pat. No. 4,414,132, has exhibited less than desirable activity in the production of polymers having an isotactic index in excess of 96 percent.
Therefore, in order to maintain satisfactorylevels of catalyst activity using the catalyst systems of U.S. Pat. Nos. 4,414,132 and 4,535,068, it is necessary to restrict the ratio of selectivity control agent (outside electron donor) to organoaluminum cocatalyst employed, as well as the temperature of polymerization. Generally, ratios no higher than 0.3:1 are employed together with temperatures no higher than 70.degree. C. The polymers prepared under such conditions have been found to have a relatively broad molecular weight distribution(M.sub.w /M.sub.n), i.e., in excess of about 5.0.
However, in order to produce polymers having a narrow molecular weight distribution, i.e., below 5.0, it is necessary to employ polymerization temperatures in excess of 80.degree. C. Polymers having a narrow molecular weight distribution and a high degree of stereoregularity are useful in applications such as fiber spinning and injection molding. To date, such polymers have not been produced directly in the polymerization reactor, but rather by post-polymerization controlled rheology techniques involving the use of peroxides to effect free radical degradation of the polymers. In order to be most feasible economically, however, any process for producing polymers of this type must be capable of producing them directly in the polymerization reactor without the necessity of postpolymerization processing by way of extraction to remove residual catalyst and/or atactic polymer produced, or rheology altering techniques.
European patent specification 0 045 977 B1 discloses a catalyst for the polymerization of alpha-olefins which comprises the reaction product of a) an alkylaluminum compound, b) a silicon compound, and c) a solid catalyst component comprising a magnesium dihalide, as essential support, and supported on said dihalide, a titanium halide or a titanium haloalcoholate and an electron donor selected from certain esters.
Die Angewandte Makromolekulare Chemie, 120 (1984) 73-90 (Nr. 1935), "High Yield Catalysts in Olefin Polymerization", Paolo Galli, Pier Camillo Barbe and Luciano Noristi further teaches that both yield and isotacticity of polymers prepared by means of certain stereospecific catalysts containing magnesium dichloride and titanium tetrachloride may be improved by increasing the temperature of polymerization from 50.degree. C. to 80.degree. C. (FIGS. 13 and 14). However, the precise nature of the catalyst, and how it is prepared, is not described in the reference, nor is the ratio of outside electron donor (or Lewis base) to alkylaluminum cocatalyst which must be employed with such catalyst in order to obtain polymers having high isotacticity in high yield
U.K. patent application 2 1110066 A teaches that catalysts similar to those of European patent specification 0 045 977 B1 can be employed to polymerize propylene at temperatures of 80.degree. C.-90.degree. C. using ratios of outside electron donor (selectivity control agent) to alkylaluminum compound of 0.05:1 to 0.1:1 (see examples 7-14) to produce high yields of polymer having a high degree of stereoregularity. The behavior of these catalysts contrasts sharply with the behavior of the catalyst systems of U.S. Pat. Nos. 4,414,132 and 4,535,068 which undergo a decline in activity as the temperature of polymerization is increased and a decline in stereospecificity as the ratio of outside electron donor to alkylaluminum cocatalyst is decreased.
Thus, it is clear from the prior art that the ratio of outside electron donor (selectivity control agent) to alkylaluminum cocatalyst present in a given catalyst system, and the polymerization temperature at which the catalyst system is employed, significantly affect both catalyst activity and the isotacticity of the polymers produced. The effect of these factors on catalyst activity and polymer isotacticity vary widely from system to system, and appear to produce divergent and contradictory results depending upon the nature of the catalyst employed and the manner in which it is prepared. The dependence of a particular catalyst system upon factors of this nature restricts the versatility of the system and limits the conditions under which it may be employed, and thus the ability of the system to produce polymers having a variety of properties at acceptable levels of catalyst activity. For example, to date no process has been proposed which is capable of producing polymers having a narrow molecular weight distribution as well as a high level of isotacticity at satisfactory levels of catalyst activity in a low pressure gas phase fluid bed process.